spl: Convert mmc to spl_load

This converts the mmc loader to spl_load. Legacy images are handled by
spl_load (via spl_parse_image_header), so mmc_load_legacy can be
omitted. To accurately determine whether mmc_load_image_raw_sector is used
(which might not be the case if SYS_MMCSD_FS_BOOT is enabled), we introduce
a helper config SYS_MMCSD_RAW_MODE. This ensures we can inline spl_load
correctly when a board only boots from filesystems. We still need to check
for SPL_MMC, since some boards enable configure raw mode even without MMC
support.

Signed-off-by: Sean Anderson <seanga2@gmail.com>
Reviewed-by: Simon Glass <sjg@chromium.org>